1. Field of the Invention
The present invention relates to image processing apparatuses and methods for printing and plate making, and more specifically to trapping technology for image data containing a translucent image (a translucent object).
2. Description of the Background Art
In the field of printing and plate making, a personal computer or the like is used to first perform an edit process based on characters that are to be contained in a print and a plurality of types of other print elements, such as logos, patterns, and illustrations, and generate page data containing a print target written in a page-description language. Thereafter, a RIP process is performed on the page data to generate image data for use in production of a plate (a press plate) that is to be placed in a printing machine.
By the way, in the case of multicolor printing, in order to prevent an underlying portion from being exposed in a boundary between colors due to register movement, a trapping process is performed before performing the RIP process on the page data. The trapping process is to dispose, along a boundary portion between two adjacent colors on an image, a hairline graphic (hereinafter, referred to as a “trap graphic”), which has a color containing color elements from both sides of the boundary portion. For example, in the case where a Y color graphic object 71, a C color graphic object 72, and an M color graphic object 73 are located so as to overlap with each other as shown in FIG. 11A, the trapping process generates trap graphics 74a to 74c in boundary portions between the overlapping graphic objects as shown in FIG. 11B.
The procedure of the trapping process is described below by taking as an example the case where the three color graphic objects 71, 72, and 73 are present within a page as shown in FIG. 11A. Note that these graphic objects are arranged in the order, from bottom (back) to top (front), the Y Color graphic object 71, the C color graphic object 72, the M color graphic object 73.
Performed first is a process for correlating each pixel within a display area with a graphic object that is to be displayed by the pixel (hereinafter, referred to as an “ID drawing process”) In the ID drawing process, each graphic object is assigned a unique identification number (hereinafter, referred to as “ID”). For example, in the case where n graphic objects are present within a page, the graphic objects are assigned IDs “1”, “2”, . . . , “n”in order from bottom (back) to (front). In the example shown in FIG. 11A, the graphic objects 71, 72, and 73 are assigned “1”, “2”, and “3”, respectively.
Once each graphic object is assigned an ID, ID drawing is performed for each pixel of the graphic objects in order from lowest ID to highest ID (i.e., in order from bottommost to topmost) In the example shown in FIG. 11A, the ID drawing is first performed for the Y color graphic object 71. This leads to a drawing result as shown in FIG. 12A. Next, the ID drawing is performed on the C color graphic object 72. In this case, for pixels in the area where the Y color graphic object 71 and the C color graphic object 72 overlap, the ID of the Y color graphic object 71 is overwritten by the ID of the C color graphic object 72. This leads to a drawing result as shown in FIG. 12B. Further, the ID drawing is performed on the M color graphic object 73. In this case also, the ID of the Y color graphic object 71 and the ID of the C color graphic object 72 are overwritten. This leads to a drawing result as shown in FIG. 12C. Thus, the ID drawing process is completed.
After the ID drawing process, a list (hereinafter, referred to as a “related graphic list”) that indicates overlapping of the graphic objects (relative positional relationships in the vertical direction) is generated. FIG. 13 is a diagram schematically illustrating a related graphic list generated in the example shown in FIG. 11A. As shown in FIG. 13, an upward direction related graphic listing and a downward direction related graphic listing are generated for each graphic object within the page. The related graphic list is generated in the following manner based on the drawing result (hereinafter, referred to as the “ID drawing result”) obtained by the ID drawing process as shown in FIG. 12C.
First, each pixel within the display area is scanned, focusing on “ID=1”. In this case, if an ID other than “1” is drawed for any pixel adjacent to a pixel for which “1” is drawed, the ID is added to the upward direction related graphic listing. In FIG. 12C, “1” is adjacent to “2” and “3”, and therefore “2” and “3”are added to the upward direction related graphic listing for the graphic object with “ID=1” as shown in FIG. 13.
Next, each pixel within the display area is scanned, focusing on “ID=2”. In this case, when an ID other than “2” is drawed for any pixel adjacent to a pixel for which “2” is drawed, the ID is added to the downward direction related graphic listing if the ID has a value less than “2” or to the upward direction related graphic listing if the ID has a value more than “2”. Accordingly, as shown in FIG. 13, “3” is added to the upward direction related graphic listing for the graphic object with “ID=2” and “1” is added to the downward direction related graphic listing for the graphic object with “ID=2”.
Further, a similar operation is performed for “ID=3”. As such, the related graphic list as shown in FIG. 13 is generated. More specifically, the upward direction related graphic listing and downward direction related graphic listing are formed of structures called “related cells”. For example, the downward direction related graphic listing for the graphic object with “ID=3” consists of a related cell for “ID=2” and a related cell for “ID=1” as shown in FIG. 14.
After the related graphic list is generated, trap attributes are set in the “related cell” structures based on predetermined setting information, and trap graphics are generated between graphic objects based on the contents of the settings. Thus, any underlying portion is suppressed from being exposed in color boundary portions due to register movement at the time of multicolor printing.
Note that Japanese Laid-Open Patent Publication No. 9-106460 discloses an example of generating the related graphic list.
By the way, in some cases, in order to give a three-dimensional appearance to graphics, an object for casting a shadow, for example, around the graphics (hereinafter, referred to as a “shadow-casting object”) is used. In such a case, there is a possibility that desired trap graphics might not be generated. This is described with reference to FIG. 15. In FIG. 15, a plurality of C color graphic objects 81, a plurality of M color graphic objects 82, and a Y color graphic object 83 are present within a page. In addition, these objects are arranged in the order, from bottom (back) to top (front), the C color graphic objects 81, the M color graphic objects 82, the Y color graphic object 83. In such a case, in order to give a three-dimensional appearance to the Y color graphic object 83, a shadow-casting object 84 may be placed below (on the backside relative to) the Y color graphic object 83 but above (on the frontside relative to) the M color graphic objects 82 (i.e., between the M color graphic objects 82 and the Y color graphic object 83) to display a shadow, while gradually reducing color density in the directions indicated by the arrows in FIG. 15.
The shadow-casting object 84 is an object visually recognized on the screen as a translucent graphic. Therefore, the graphic objects 81 and 82 located below (on the backside relative to) the shadow-casting object 84 are also visually recognized on the screen. However, data for the object covering the area denoted by reference numeral 84 in FIG. 15 is present. Therefore, the graphic objects 81 and 82, which are present, for example, in the area denoted by reference numeral 86 in FIG. 15, are completely included in the area of the shadow-casting object 84. Accordingly, at the time of the above-described ID drawing process, the IDs of the graphic objects located below (on the backside relative to) the shadow-casting object 84 are overwritten by the ID of the shadow-casting, object 84.
For example, in the case where three graphic objects 91 (ID=1), 92 (ID=2) and 93 (ID=3) and a shadow-casting object 94 (ID=4) are present as shown in FIG. 16, and they are arranged in the order, from bottom (back) to top (front), the graphic object 91, the graphic object 92, the shadow-casting object 94, the graphic object 93, the ID drawing result is as shown in FIG. 17. Although the graphic object 91 is visually recognized on the screen, the ID of the graphic object 91 is overwritten by the ID of the shadow-casting object 94 as shown in FIG. 17.
Accordingly, for the graphic objects located below (on the backside relative to) the shadow-casting object 84 in the example shown in FIG. 15, positional relationships relative to other objects are not acquired, and therefore the trapping process leads to results as shown in FIGS. 18A and 18B. FIG. 18A shows the result for the area denoted by reference numeral 85 in FIG. 15, and FIG. 18B shows the result for the area denoted by reference numeral 86 in FIG. 15. In the area outside the shadow-casting object 84, trap graphics are generated at boundary portions between the graphic objects as shown in FIG. 18A. In the area of the shadow-casting object 84, on the other hand, no trap graphic is generated as shown in FIG. 18B.
Thus, when the shadow-casting object is contained in page data, there is a possibility that desired trap graphics might not be generated.